1. Field of the Invention
The present invention relates to three-dimensional photonic crystals operating at a plurality of wavelengths. The present invention particularly relates to a three-dimensional photonic crystal which has a point defect serving as a resonator and/or a linear defect serving as a waveguide and which operates at desired wavelengths and also relates to an optical device including such a three-dimensional photonic crystal.
2. Description of the Related Art
Yablonovitch has introduced the idea of controlling the transmission and reflection of electromagnetic waves using fine structures having dimensions smaller than the wavelengths thereof (Physical Review Letters, Vol. 58, pp. 2059 (1987)). That is, the transmission and reflection of an electromagnetic wave can be controlled with an array of the fine structures. If the wavelength of the electromagnetic wave is as short as the wavelength of light, the transmission and reflection of light can be controlled with the fine structures. Materials having the fine structures are known as photonic crystals. A reflecting mirror that has a reflectivity of 100%, that is, no optical loss, over a certain range of wavelengths can be probably prepared using a photonic crystal. Such a range of wavelengths is referred to as “photonic band gap (PBG)” analogous to the energy gap of known semiconductors. A three-dimensional periodic fine structure has a photonic band gap in which light cannot propagate in any direction. Such a photonic band gap is hereinafter referred to as “complete photonic band gap”. Structures with a complete photonic band gap can be used for various applications, for example, the control of the spontaneous emission of light. This leads to the possibility of novel functional devices. Thus, there is an increasing demand for functional devices having structures with complete photonic band gaps over a wide wavelength range.
U.S. Pat. No. 5,335,240 and 6,597,851 disclose some structures with photonic band gaps. Examples of a three-dimensional periodic structure include structures shown in FIGS. 25A to 25E. FIGS. 25A, 25B, 25C, 25D, and 25E show a diamond opal structure, a wood-pile structure, a spiral structure, a unique three-dimensional structure, and a structure inverse to the three-dimensional structure, respectively.
If a defect is formed in a periodic fine structure with a photonic band gap, the periodic fine structure can be used as a resonator or waveguide for emitting or guiding, respectively, light with a desired wavelength. If the defect is point-shaped or linear, the periodic fine structure serves as a point-defect resonator or a linear defect waveguide. Such a point-defect resonator with a photonic band gap can confine light in a small region of space; hence, a light-emitting device including the point-defect resonator can emit light with any wavelength with high efficiency and the light emission of the light-emitting device can be precisely controlled. As discussed in U.S. Pat. No. 5,784,400, laser oscillation can be achieved in such a manner that a point defect is formed in a periodic structure and then filled with a luminous material and light is emitted from the luminous material using an excitation unit.
U.S. Pat. No. 5,406,573 and other documents disclose various methods for manufacturing photonic crystals.
Photonic band gaps can be controlled by varying the grating periods of photonic crystals. An increase in grating period shifts a photonic band gap to longer wavelengths and a decrease in grating period shifts a photonic band gap to shorter wavelengths.
Nature, vol. 407, p. 608 (2000) reports that the operating wavelength of an optical add/drop multiplexer (an optical add/drop circuit) including a two-dimensional photonic crystal is controlled by varying the grating period. The optical add/drop multiplexer is an optical input/output circuit that adds a new wavelength to a medium in which a plurality of wavelengths propagate or that removes (drops) a specific wavelength from such a medium. Small-sized optical add/drop multiplexers can probably be manufactured using photonic crystals. Nature, vol. 407, p. 608 (2000) also reports that substantially equal drop efficiencies can be achieved for a plurality of wavelengths in such a manner that a waveguide and a resonator are tuned to desired operating wavelengths by varying the grating period of the two-dimensional photonic crystal. A two-dimensional photonic crystal with different grating periods is referred to as an in-plane hetero photonic crystal and is a good example showing that the control of a photonic band gap is a key to develop an optical nanodevice including a photonic crystal.
Structures with different grating periods cannot be directly applied to three-dimensional photonic crystals. A photonic band gap of a three-dimensional photonic crystal can be controlled by varying the grating period thereof. This causes structural displacement between regions with different grating periods as illustrated in FIG. 26. In three-dimensional structures, structural displacement occurs in the X-, Y-, and Z-axis directions; hence, the manufacture of three-dimensional structures with no structural displacements is difficult. In a layer-by-layer structure including stacked layers with a wood-pile structure, the grating period thereof varies in the stacking direction of the layers. Therefore, it is difficult to manufacture such a layer-by-layer structure by the following conventional processes: a wafer fusion process, a nanoimprinting process, and a process in which layers patterned by electron beam lithography are stacked.